1. Field of the Invention
The present invention relates to a filtering device used in a high-frequency device for use in a mobile communication system or the like.
2. Description of the Related Art
As a result of recent introduction of the TMDA technique into portable telephone systems, the communication scheme of intermittent transmission/reception in units of time slots has become widely used instead of the concurrent transmission/reception technique. As a result of the change in the communication scheme, the microwave filter which is located at the first stage of a radio communication device and which is used in common in transmission and reception has been changed from a combination of transmission and reception filters to a switching type filter in which a transmission filter and a reception filter are switched from time to time.
In general, when a transmission filter and a reception filter are switched from each other by a switch, isolation of the switching circuit makes it possible to reduce signal leakage from a transmission circuit to a reception circuit to a lower level than can be achieved by a single filter. Therefore, requirement of the attenuation characteristic for a filter of the transmission-reception switched type is less severe than that for a filter of the combined transmission-reception type. This makes it possible to realize a smaller-sized filter at a lower cost.
FIG. 31 illustrates a typical transmission-reception switched type filter. In FIG. 31, diodes D1 and D2 are used as switching devices for switching a transmission filter and a reception filter from each other. If a switching control current is applied so as to turn on both diodes D1 and D2 into a closed state, a transmission signal is passed through the transmission filter to an ANT terminal. However, because the transmission signal is shunted to ground by the diode D2, the transmission signal cannot reach the reception filter. On the other hand, when the switching control signal is given in such a manner as to turn off both diodes D1 and D2 into an open state, a reception signal is passed through the reception filter. In FIG. 31, L3 is a high-frequency choke coil and C2 is a high-frequency signal shunting capacitor. The combination of L3 and C2 prevents ingress of the RF signal to a control circuit which generates the switching control signal.
To improve the isolation of the switching circuit using diodes, it is more desirable to dispose the diodes in a shunted fashion. If the diodes are disposed in a series fashion, leakage of signal occurs due to residual capacitance when the diodes are in an off-state, which results in degradation in isolation between reception and transmission filters.
However, in the switching circuit of the type in which a switching device is turned on into a closed state so as to shunt the circuit, it is required that the impedance of the switching device seen from the antenna terminal should be as high as can be regarded as open-circuited thereby eliminating the influence of the closed switching device on the filter used. One known technique of achieving the above requirement is to add an LC phase shift circuit consisting of L1, L2, and C1 to the switching device as shown in FIG. 31. Another technique is to insert a xcexg/4 transmission line so that the impedance seen from the transmission filter becomes as high as can be regarded as substantially open-circuited.
Thus, it is an object of the present invention to provide a filtering device of the transmission-reception switched type which can be constructed in a form with a reduced size at a low cost without having to use circuit elements such as a capacitor and a coil forming a phase shift circuit which are not essential to the filtering device.
To achieve the above requirement of reducing the device size and the production cost without using a conventional phase shift circuit, the present invention provides a filtering device according to any aspect described below. According to a first aspect of the present invention, there is provided a filtering device comprising: a plurality of filters each having a distributed-parameter resonance line at least one end of which is open-circuited; and a coupling line, a coupling electrode, or a coupling element coupled to at least one distributed-parameter resonance line included in each filter, wherein a switch is connected to the above-described at least one distributed-parameter resonance line so that the open-circuited end of the above-described at least one distributed-parameter resonance line is short-circuited when the switch is operated.
FIG. 1 illustrates a specific example of the circuit configuration of the filtering device according to the above aspect of the invention. As shown in FIG. 1, the filtering device comprises: distributed-parameter resonance lines R11, R12, R13, R21, R22, and R23 whose one end is open-circuited; and coupling reactances k11, k12, k13, k14, k21, k22, k23, and k24 located between adjacent distributed-parameter resonance lines or between an input or output port and a first- or final-stage line. In this specific example, a filter 1 is formed between port 1 and port 3 and a filter 2 is formed between port 3 and port 2. Diode switches (hereinafter referred to simply as switches) D1 and D2 are connected between the open-circuited ends of the distributed-parameter resonance lines R13 and R21 and ground. Although a bias circuit for supplying a bias voltage to the switches D1 and D2 are needed, it is not shown in FIG. 1. The direction of the switches D1 and D2 is not limited to that shown in FIG. 1, but the direction may be determined in different manners depending on the configuration of the bias circuit used to supply a bias voltage to the switches D1 and D2.
In FIG. 1, when the switch D2 is in an open state and the switch D1 is in a closed state, the distributed-parameter resonance line R13 is short-circuited at its both ends, and thus it acts as a xcex/2 resonator. In this state, the other distributed-parameter resonance lines act as xcex/4 resonators and therefore they have a resonance frequency twice the signal frequency. As a result, the distributed-parameter resonance line R13 acts as a very high impedance (very low admittance) at frequencies in the signal frequency band. In this state, on the other hand, the coupling reactance k14 between the distributed-parameter resonance line R13 and the port 3 acts as an impedance directly connected to ground via the switch D1. Therefore, when seen from the port 3, the filter 1 is not short-circuited but it is seen as a circuit having a certain reactance. If the filter 2 is designed taking into account this reactance, the filter 2 can have desired characteristics independent of the filter 1. In the case where the filter 2 operates using the port 3 as an input port and the port 2 as an output port, when the switch D1 is in a closed state, a signal input to the port 3 is passed through the filter 2 and output to the port 2 but no signal is output to the port 1. On the other hand, in the case where the filter 2 operates using the port 2 as an input port and the port 3 as an output port, when the switch D1 is in a closed state, a signal input to the port 2 is passed through the filter 2 and output to the port 3, but no signal is input to the filter 1.
Conversely, if the switch D1 is in an open state and the switch D2 is in a closed state, the filter 1 can be used without being affected by the filter 2.
In the design of the filter, when the filter 2 is designed first so that the filter 2 has desired characteristics taking into account the effects of k14. This can be achieved by performing a simulation repeatedly on the filter 2 taking into account the reactance k14 while varying parameters of the respective elements in the filter 2 by small amounts at a time until desired characteristics are achieved. As a result, optimized parameters of the filter 2 are obtained, and thus the optimized value for the coupling reactance k21 between the port 3 and the distributed-parameter resonance line R21 is determined. This value for k21 is fixed, and the optimized parameters of the filter 1 located on the opposite side are determined by performing a simulation repeatedly while varying the parameters of the respective elements in the filter 2 by small amounts at a time.
In the above example, when the switch is turned on into a closed state, the xcex/4 resonator one end of which is open-circuited and the other end of which is short-circuited is converted to a xcex/2 resonator both ends of which arc short-circuited. Alternatively, the filtering device may also be constructed such that when a switch is turned on into a closed state, a xcex/2 resonator whose both ends are open-circuited may be converted to a xcex/4 resonator one end of which is open-circuited and the other end of which is short-circuited. In this case, when the switch is turned on, the resonance frequency becomes times the signal frequency, and thus the distributed-parameter resonance line acts as a very high impedance at frequencies in the signal frequency band.
In the above-described filtering device, when the switch is in an open state, the distributed-parameter resonance line connected to the switch operates in a normal mode. Alternatively, the distributed-parameter resonance line connected to the switch may operate in a normal mode when the switch is in a closed state. That is, according to a second aspect of the present invention, there is provided a filtering device comprising: a plurality of filters each having a distributed-parameter resonance line at least one end of which is short-circuited; and a coupling line, a coupling electrode, or a coupling element coupled to at least one distributed-parameter resonance line included in each filter, wherein a switch is connected to the above-described at least one distributed-parameter resonance line so that the short-circuited end of the above-described at least one distributed-parameter resonance line is open-circuited when the switch is operated. In this configuration, in the case where the other end of the distributed-parameter resonance line is short-circuited, when the switch is turned off into an open state, the xcex/2 resonator both ends of which are short-circuited is changed to a xcex/4 resonator one end of which is short-circuited and the resonance frequency becomes 1/2 times the original resonance frequency. On the other hand, in the case where the other end of the distributed-parameter resonance line is open-circuited, when the switch is turned off into an open state, the xcex/4 resonator one end of which is short-circuited is changed to a xcex/2 resonator both ends of which are open-circuited, and the resonance frequency becomes 2 times the original resonance frequency. In either case, when the switch is turned off into the open state, the distributed-parameter resonance line comes to behave as a very high impedance, and therefore the filter connected to the opened switch can be substantially isolated from the other filter.
A filtering device may also be constructed, according to a third aspect of the invention corresponding to claim 3, using a plurality of filters each including a distributed-parameter resonance line both ends of which are short-circuited, in such a manner that a switch is connected to a substantially central part of the distributed-parameter resonance line so that the substantially central part is selectively short-circuited when the switch is operated. In this configuration, when the switch is in an open state, the distributed-parameter resonance line acts as a xcex/2 resonator both ends of which are short-circuited. When the switch is turned on into a closed state, the center of the distributed-parameter resonance line is short-circuited, and, as a result, the effective length of the resonance line becomes half the original length. As a result, the resonance frequency becomes twice the original resonance frequency, and the distributed-parameter resonance line behaves as a very high impedance at frequencies in the signal frequency band.
According to a fourth aspect of the invention, there is provided a filtering device including a plurality of filters each composed of a distributed-parameter resonance line, wherein a switch is connected to one of the distributed-parameter resonance lines located at the first stage counted from a coupling line, coupling electrode, or coupling element, so that when the switch is operated a predetermined filter becomes negligible or comes to behave as merely a reactance seen from the coupling line or coupling electrode coupled to the distributed-parameter resonance lines of each filter.
The structure of the filtering device is not limited to an integral structure such as that described above, but it may also be constructed in such a manner that a plurality of filters constructed in a separate fashion are connected to a common port via a transmission line such as a microstrip line. In this case, a switch may be connected to a distributed-parameter resonance line at the first stage counted from that common port. The number of coupling lines or coupling electrodes sharing the input/output terminal it not limited to one. For example, in the case where an antenna terminal ANT1 is used in common in both transmission and reception, and an RX terminal is used in common to output a reception signal which is received by either of two antenna terminals ANT1 and ANT2 and is transferred to the RX terminal after being passed through either of two RX filters, switches D1 and D2 may be connected to distributed-parameter resonance lines R13 and R21, respectively, at the first stage counted from the terminal ANT1, and switches D3 and D4 may be connected to distributed-parameter resonance lines R22 and R32, respectively, at the first stage counted from the terminal RX. In this configuration, when a signal is transmitted, the switch D2 is turned on so that the signal to be transmitted is prevented from reaching RX or ANT2. When a signal is received, the switch D3 is turned on so that the signal received by ANT2 is transferred to the terminal RX via the RX filter 2 or otherwise the switch D4 is turned on so that the signal received by ANT1 is transferred to the terminal RX via the RX filter 1. By properly controlling the above switching operation, antenna diversity can be achieved.
Furthermore, the above technique of the invention may also be applied to a filtering device in which one port is used in common as an input/output port by thee or more filters as shown in FIG. 4. In this case, switches D1, D2, and D3 are connected to distributed-parameter resonance lines R11, R21, and R31, respectively, at the first stage counted from port 4.
In the case where a filter at a certain location relative to a coupling line or coupling electrode is isolated so that it does not act as a filter as is the case in the above-described examples, a switch is connected to a distributed-parameter resonance line located at the first stage counted from the coupling line or coupling electrode. Alternatively, according to a fifth aspect of the invention, a switch may be connected to an open-circuited end of one of the distributed-parameter resonance lines located at the second stage counted from the coupling line or coupling electrode so that the filter characteristics can be switched by controlling the switch. In the example shown in FIG. 5, when switch D1 is in an open state, a filter 1 acts as a bandpass filter consisting of three stages of resonators realized by distributed-parameter resonance lines R11, R12, and R13. If the switch D1 is turned off; the open-circuited end of the distributed-parameter resonance line R1 is grounded via a reactance k12, and thus the distributed-parameter resonance line R11 and a coupling reactance kill comes to act as an one-stage trap circuit (bandstop filter). As a result, in this state, the filtering device acts as a bandpass filter consisting of a filter 2 formed between the port 1 and the port 2 and the one-stage trap circuit.
According to a sixth aspect of the invention, there is provided a filtering device in which at least one distributed-parameter resonance line of those forming a plurality of filters is shared by the plurality of filters, and a coupling line, coupling electrode, or a coupling element is coupled with that distributed-parameter resonance line shared. For example, as shown in FIG. 6, a distributed-parameter resonance line R3 is used in common, and one filter is formed by three stages of resonators realized by distributed-parameter resonance lines R11, R12, and R3 while another filter is formed by three stages of resonators realized by distributed-parameter resonance lines R21, R22, and R3. In this case, switches D1 and D2 are connected to the distributed-parameter resonance lines R12 and R22, respectively, at the second stage counted from the port 3. When the switch D1 is in a closed state, a reactance k31 is connected between the open-circuited end of the distributed-parameter resonance line R3 and ground. In this state, parameters are determined so that the filter formed by R21, R22, and R3 has desired characteristics. On the other hand, when the switch D2 is in a closed state, a reactance k23 is connected between the open-circuited end of the distributed-parameter resonance line R3 and ground. In this state, parameters are determined so that the filter formed by R11, R12, and R3 has desired characteristics.
Referring now to FIGS. 7(A), 7(B), 8(A) and 8(B), examples of circuits for supplying a bias voltage to diode switches will be described below.
In the example of a bias voltage supply circuit shown in FIG. 7(A), a DC blocking capacitor Cc is connected in series to a diode switch D and both ends of the diode switch D are connected to respective RF choke circuits each consisting of an inductor L and a capacitor CB. If a bias voltage is applied between terminals TB and TB so that the diode D is biased in a forward direction, then the diode D is turned on into a closed state and thus the path between terminals T1 and T2 becomes conductive for a high-frequency signal. In the example shown in FIG. 7(B), a DC blocking capacitor Cc is connected to one end of a diode switch D and the other end of the diode switch is grounded. Furthermore, an RF choke circuit consisting of an inductor L and a capacitor CB is also connected to the one end of the diode D. If a bias voltage is applied to the diode D via a terminal TB, a terminal T is grounded (short-circuited) for a high-frequency signal.
In the example shown in FIG. 8(A), a bias voltage is applied selectively to either one of terminals TB1, and TB2 so as to turn on either one of switches D1 and D2. In the example shown in FIG. 8(B), if a positive bias voltage is applied to a common terminal TB, then a switch D1 is turned on. Conversely, if a negative bias voltage is applied to the common terminal TB, then a switch D2 is turned on.
The filtering device according to any of aspects of the described above may be realized, in accordance with a seventh aspect of the invention, by using a plurality of inner conductors each acting as a distributed-parameter resonance line formed in one or more dielectric blocks.
The filtering device according to any of aspects of the invention may also be realized, in accordance with an eighth aspect of the invention corresponding to Claim 8, by using a plurality of dielectric coaxial resonators each acting as a distributed-parameter resonance line.
According to a ninth aspect of the invention, an inner conductor is formed on the inner surface of a hole in a dielectric block or in a dielectric coaxial resonator, and the switch described above is disposed inside the hole or on an opening end of the hole thereby disposing the switch in an integral fashion on the filtering device.
According to a tenth aspect of the invention, an element for supplying a bias voltage to the switch is disposed together with the switch inside the hole or on the opening end of the hole. This allows the bias voltage supply circuit to be also integrated on the filtering device.
According to a eleventh aspect of the invention, microstrip lines formed on a dielectric plate are employed as the distributed-parameter resonance lines, and a switch is disposed on the dielectric plate. This makes it possible to realize a filtering device on which the switch is integrated.
According to a twelfth aspect of the invention, an element for supplying a bias voltage to the switch is disposed on the dielectric plate. This makes it possible to realize a filtering device on which the bias voltage supply circuit is also integrated.